Such transesterification process is for example disclosed in WO2008090108, which discloses a process for recovering dialkyl carbonate from a reaction mixture obtained from reacting an alkylene carbonate and an alkanol. It has been found that in the dialkyl carbonate produced and separated in the way as described in WO2008090108, an alkoxy alkanol impurity may be present. For example, in a process where ethylene carbonate is reacted with ethanol, the products are diethyl carbonate and monoethylene glycol. However, in such process, 2-ethoxyethanol may also be formed and end up in the diethyl carbonate as an impurity.
In addition, JP2003300917 and JP2002371037 relate to processes wherein dimethyl carbonate and monoethylene glycol are made from ethylene carbonate and methanol and wherein 2-methoxyethanol is formed as a by-product, which is also an alkoxy alkanol. In the inventions of JP2003300917 and JP2002371037, said 2-methoxyethanol can be removed by specific distillation techniques.
Within the process producing an alkanediol and a dialkyl carbonate from an alkylene carbonate and an alkanol, the alkoxy alkanol impurity may be formed in various ways. For example, in a reactor where ethanol and ethylene carbonate are reacted into diethyl carbonate and monoethylene glycol, a side-reaction of ethanol with ethylene oxide, formed by back-reaction of ethylene carbonate into ethylene oxide and carbon dioxide, into 2-ethoxyethanol (ethyl oxitol) may take place. Further, ethyl oxitol may be formed by a side-reaction of ethanol with ethylene carbonate in such a way that carbon dioxide is released and ethyl oxitol is produced. Still further, a side-reaction between ethanol and monoethylene glycol may take place producing ethyl oxitol and water. Still even further, ethyl oxitol may be formed via decarboxylation of hydroxyethyl ethyl carbonate.
Therefore, the product stream from a reactor where ethanol and ethylene carbonate are reacted into diethyl carbonate and monoethylene glycol, may comprise unconverted ethanol, unconverted ethylene carbonate, diethyl carbonate, monoethylene glycol and the above-mentioned ethyl oxitol impurity. The presence of said alkoxy alkanol impurity may be detrimental in any subsequent production process. Said alkoxy alkanol impurity may for example end up in the dialkyl carbonate that is used as a starting material for the synthesis of diphenyl carbonate from said dialkyl carbonate and phenol. For example, in a case where the dialkyl carbonate is diethyl carbonate and the alkoxy alkanol impurity is ethyl oxitol, said ethyl oxitol may react with the phenol starting material and/or with the diphenyl carbonate product.
Direct reaction of phenol and ethyl oxitol may result in the production of phenyl 2-ethoxyethyl ether, and hence in the loss of valuable phenol reactant. Further, such reaction results in the introduction of undesired chemicals in the process and therefore to separation issues.
Reaction of diphenyl carbonate with ethyl oxitol results in product loss as phenyl 2-ethoxyethyl carbonate is produced. Further, the latter product acts as a “poison” in any subsequent polymerisation of diphenyl carbonate into polycarbonate material. For example, when diphenyl carbonate is reacted with bis-phenol A (BPA), polycarbonate and phenol are formed. Diphenyl carbonate can react with BPA since phenol is a relatively good leaving group. Dialkyl carbonates (such as diethyl carbonate) however cannot be used to produce polycarbonate by reaction with BPA, since alkanols are not good leaving groups. Alkoxy alkanols (such as ethyl oxitol) are neither good leaving groups. Therefore, in case phenyl 2-ethoxyethyl carbonate is present in a diphenyl carbonate feed to be reacted with BPA, phenol will be released easily from said phenyl 2-ethoxyethyl carbonate but not ethyl oxitol which will consequently stop the polymerization process at one end of the chain. Consequently, phenyl 2-ethoxyethyl carbonate should be removed from diphenyl carbonate before the latter is contacted with BPA.
The above exemplifies that in a case where a dialkyl carbonate stream containing an alkoxy alkanol impurity is formed, it is desired to remove said alkoxy alkanol impurity before any subsequent process takes place wherein the dialkyl carbonate is transformed into a valuable end product. For example, it is desirable to remove any ethyl oxitol impurity from a diethyl carbonate stream containing said impurity before reaction of the diethyl carbonate with phenol takes place.
Referring to the above example where ethanol and ethylene carbonate have been reacted into diethyl carbonate and monoethylene glycol, the product stream also containing unconverted ethanol and ethylene carbonate and ethyl oxitol side-product, may be separated by means of distillation. The boiling points for the various components in said product stream are mentioned in the table below.
ComponentBoiling point (° C.)ethanol78.4diethyl carbonate126-128ethyl oxitol135monoethylene glycol197.3ethylene carbonate260.4
The distillation as referred to above may result in a top stream containing diethyl carbonate and unconverted ethanol and a bottom stream containing monoethylene glycol and unconverted ethylene carbonate. Ethyl oxitol may end up in said top stream as an impurity. Subsequently, said top stream may be further separated by means of distillation into a top stream containing unconverted ethanol which can be recycled to the reactor where diethyl carbonate and monoethylene glycol are produced, and a bottom stream containing diethyl carbonate and the ethyl oxitol impurity.
As discussed above, before a dialkyl carbonate is transformed into a valuable end product in any subsequent process, the alkoxy alkanol impurity has to be removed therefrom as that might interfere said subsequent process and/or any further processes. For the above example, this means that the ethyl oxitol impurity should be removed from the bottom stream containing diethyl carbonate and the ethyl oxitol impurity. In principle, ethyl oxitol and diethyl carbonate could be separated by means of a further distillation step. However because of the small difference in boiling point between diethyl carbonate and ethyl oxitol (see above table), such separation is very cumbersome requiring many distillation steps and stages.
Therefore, there is a need to find a simple method of removing an alkoxy alkanol impurity from a dialkyl carbonate stream containing such alkoxy alkanol impurity, and at the same time to prevent a decrease of the overall yield of dialkyl carbonate as much as possible.